The invention relates to equipment and a method for removing an unwanted noise signal and harmonics thereof from electric signals obtained through a measuring operation, such as a seismic operation.
In particular, the invention relates to equipment and a method for removing a sinusoidal noise signal of substantially constant frequency and amplitude from a measured electric signal. Such type of noise signal often results from the occurrence of stray electric currents at the site where the measurements are being carried out. The noise is a particular nuisance where low-energy signals are being measured, or when taking measurements in the vicinity of industrialized areas where high-energy power lines and electric equipment are present, or in the neighborhood of electric railway tracks. The magnetic fields created by such currents induce an electric noise signal in the measuring equipment and electric cables attached thereto. The noise signal is recorded together with the signals resulting from the measuring operation and it is not possible to remove the noise signal from the recorded signals by known filtering methods without, at the same time, degrading the signals that are to be measured.
Apart from the electric noise signal resulting from alternating magnetic fields, electric stray currents can be expected to enter the measuring system directly. This will occur where measuring cables are in contact with moisture-containing earth as often happens when electric cables having low-quality electric insulation are used in seismic measuring operations. These stray currents are detected together with the signals to be measured, thereby reducing the quality thereof.
It has been proposed earlier to remove sinusoidal noise signals of an unvarying frequency f from seismic records by compounding N records resulting from seismic waves initiated at time intervals K/f + 1/Nf (wherein K is an integer). Compounding these records will, in theory, remove the sinusoidal noise signal by an integration operation (since--as known--addition of N sinusoidal voltages of the same amplitude and frequency, and phase-shifted with reference to one another by an angle equal to 4 .pi./N produces a resultant equal to zero).
Such integration technique for suppressing a sinusoidal noise signal, however, has only been found suitable to partially remove the noise signals that seismic operators have to face in the field.